Telomerase serves essential functions in countering the end-replication problem and conferring immortal replicative potential upon cancer cells. Inhibition of telomerase has been shown in model systems to block tumor growth and has therefore been considered a promising anti-cancer therapeutic strategy. Successful implementation of anti-telomerase therapy has been hampered by an incomplete understanding of the telomerase enzyme and its mechanisms of action. Telomerase is regulated through a complex, yet essential assembly and trafficking pathway that culminates in recruitment of the enzyme to its physiological substrate, the chromosome end. Telomerase assembly requires specific assembly factors to produce an enzymatically active ribonucleoprotein comprising the telomerase reverse transcriptase (TERT), the telomerase RNA component (TERC) and dyskerin, a TERC-binding protein required for telomerase stability. In the final steps in the pathway, the telomerase protein TCAB1 enables telomerase accumulation within Cajal bodies, sites of ribonucleoprotein assembly and modification. After this step, telomerase is able to translocate to telomeres through a crucial, but incompletely understood process. This proposal studies the mechanisms by which telomerase associates with telomeres in human cancer cells. We dissect the association between telomerase and TPP1, a telomere-binding protein that serves roles both in protecting chromosome ends and in controlling telomerase access to telomeres. Telomerase can interact with the oligonucleotide/oligosaccharide binding fold (OB-fold) of TPP1 and we develop new assays that allow the interaction between telomerase and the OB-fold of TPP1 to be studied in vivo. We find that the OB-fold acts as a potent inhibitor of telomerase action at telomeres, inhibiting telomerase recruitment and causing rapid telomere shortening. Insights gained from understanding how telomerase is recruited to telomeres may enable the generation of targeted therapeutics that block the ability of telomerase to support immortal growth of tumors. To understand how telomerase is recruited to telomeres in human cancer cells, we will pursue the following specific aims: (1) To define the interaction between telomerase and TPP1 (2) To understand telomerase recruitment through real-time live cell imaging and (3) To determine the genetic requirement for TPP1-telomerase association in telomere maintenance.